Side seam-coated welded cans and process for preparation thereof

ABSTRACT

Disclosed is a side seam-coated welded can comprising a welded can body having a seam on the side face and a resin coating layer covering at least one surface side of the seam, wherein said coating layer comprises a thermosetting resin and a thermoplastic resin having a softening point of 50° to 300° C. as determined according to the ring and ball method, at a volume ratio ranging (A) from 95/5 to 25/75 or (B) from 20/80 to 1/99, and one of said two resins is present in the coating in the form of a continuous phase and at least a part of the other resin is present in the coating in the form of fine dispersed particles. 
     This can is excellent in corrosion resistance and processability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 164,148, filed June 30,1980, now U.S. Pat. No. 4,382,525.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a welded can having a coated side seamand a process for the preparation thereof. More particularly, theinvention relates to a welded can excellent in the corrosion resistanceand processability, in which a coating of a thermosetting resin and athermoplastic resin is formed on a seam by one step and in this coating,one of the two resins is present as a continuous phase and the otherresin is present as a dispersed phase, and also to a process for thepreparation of such can.

(2) Description of the Prior Art

As the conventional process most popularly adopted for the manufactureof can bodies, there can be mentioned a process comprising forming ametallic can blank cut in a predetermined size into a cylinder, lappingboth the edges of the blank and bonding the lapped edges to form a sideseam by welding, soldering or using an adhesive.

In a can body prepared according to this process, the cut edge of theblank is exposed on the inner face side of the can body, and in order toprevent corrosion of the can blank and control dissolution of the metalinto the packed content, it is very important to coat this exposed cutedge.

Various processes for coating and protecting side seams, especially cutedges, have been heretofore proposed. As the process capable ofattaining considerable effects, there can be mentioned a process inwhich the cut edge of a can blank is coated and protected in advancewith a tape of an adhesive such as a polyamide. This process can beapplied to the manufacture of bonded seam cans, but this process cannotbe practically utilized if the seam is exposed to a very hightemperature as in the case of welded seam cans.

As means for coating and protecting the side seam of a welded seam can,there is known a process in which a paint in the form of a solution orpowder is coated on the inner or outer face side of the seam afterformation of a can body. However, known paints used for this purpose areinsufficient in combination of the adhesion to the side seam, thebarrier property against corrosive components and processability, forexample, adaptability to the double-seaming operation, and the object ofcompletely coating the stepped cut edge in the side seam cannot besatisfactorily attained by these known paints.

For example, a coating material comprising a thermoplastic resin hasexcellent processability, but the thermoplastic resin is poor in theadhesion to the side seam and insufficient in the above-mentionedbarrier property. Accordingly, a side seam coated with a thermoplasticresin is readily corroded by the content or the like and defects such assulfide blackening are caused.

A plant comprising a thermosetting resin is excellent in the adhesion tothe side seam and the barrier property to corrosive components, but hasinferior processability. In a can body having a side seam coated with athermosetting resin, dissolution of the metal from the side seamedportion is often observed.

When these known paints are used, whether they comprise a thermosettingresin or a thermoplastic resin, they flow in the form of a solution orin the molten state so that the stepped portion of the cut edge islevelled. Therefore, the coating is not present in the angular portionof the cut edge or the thickness of the coating is extremely reduced inthis portion. Furthermore, air bubbles are readily included in thecoating of the stepped portion. Thus, it is substantially impossible tocoat the cut edge of the can blank completely.

If there is present an exposed metal portion in a can body, a defectivecan such as a hole-pierced can or a hydrogen-swollen can is formedaccording to its content, and furthermore, the flavor of the content isdrastically degraded.

SUMMARY OF THE INVENTION

We found that when a paint comprising a solution of a thermosettingresin as the dispersion medium and particles of a specific thermoplasticresin as the dispersed phase and containing the thermosetting resin andthe thermoplastic resin at a volume ratio as solids ranging (A) from95/5 to 25/75 or (B) from 20/80 to 1/99 is applied to the seam of awelded can, it is possible to completely coat even the angular portionof the cut edge present on the side seam and perform the coating andbaking operations without bubbling. It also was found that if thiscoating process is adopted, there can be formed a coating in which,according to the above-mentioned volume ratio, one of the thermosettingresin and the thermoplastic resin is present in the form of a continuousphase and at least a part of the other resin is present in thecontinuous phase in the form of fine dispersed particles, and that thecoating having this specific dispersion characteristic is excellent inthe combination of adhesion, corrosion resistance and processability.

It is therefore a primary object of the present invention to provide aside seam-coated welded can in which the side seam portion of the weldedcan is entirely coated completely and this coating has excellentadhesion, corrosion resistance and processability and also provide aprocess for the preparation of this side seam-coated welded can.

Another object of the present invention is to provide a welded can inwhich the welded seam is effectively protected with a coating in which athermosetting resin and a thermoplastic resin are present in a specificdispersion state, and also provide a process for the preparation of thiscan.

Still another object of the present invention is to provide a processfor the preparation of side seam-coated welded cans in which a piece ofconventional coating equipment that has been used for the coatingoperation using known solution type paints can be used without anyparticular modification and a thick coating free of bubbles or foams canbe formed entirely on the side seam of a welded can.

In accordance with one fundamental aspect of the present invention,there is provided a side seam-coated welded can comprising a welded canbody having a seam on the side face and a resin coating layer coveringat least one surface side of the side seam, wherein said coating layercomprises a thermosetting resin and a thermoplastic resin having asoftening point of 50° to 300° C. as determined according to the ringand ball method, at a volume ratio ranging (A) from 95/5 to 25/75 or (B)from 20/80 to 1/99, and one of said two resins is present in the coatingin the form of a continuous phase and at least a part of the other resinis present in the coating in the form of fine dispersed particles.

In accordance with another fundamental aspect of the present invention,there is provided a process for the preparation of side seam-coatedwelded cans, which comprises applying a resin paint to a welded can bodyhaving a seam formed on the side face thereof at least on one surface ofsaid seam and baking the coated paint to form a coating covering saidside seam, wherein said resin paint comprises a dispersion mediumcomposed of a solution of a thermosetting resin and a dispersed phase ofresin particles having a number average particle size of 0.1 to 80 μmand being composed of a thermoplastic resin having a softening point of50° to 300° C. as determined according to the ring and ball method, thevolume ratio of the thermosetting resin to the thermoplastic resin inthe coating ranges (A) from 95/5 to 25/75 or (B) from 20/80 to 1/99, andthe paint coated on the side seam is baked under such conditions thatevaporation of a solvent in the solution of the dispersion medium isfirst caused and softening of the thermoplastic resin is then caused.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged sectional view showing the main part of oneembodiment of the side seam-coated welded can according to the presentinvention.

FIG. 2 is an enlarged sectional view showing the main part of anotherembodiment of the side seam-coated welded can according to the presentinvention.

FIG. 3 is an enlarged sectional view showing the surface portion of aside seam formed on a welded can of tinplate.

FIG. 4 is an enlarged sectional view showing the side seam andperipheral surface portion of a welded can of tin-free steel (TFS).

In the drawings, reference numerals 1, 2, 5, 6, 7, 8, 9, 11, 14, 15 and21 represent a metallic can blank, a side seam, a resin coating layer, acontinuous phase of a thermosetting resin, thermoplastic resinparticles, a continuous phase of a thermoplastic resin, a continuousthin layer of a thermosetting resin, dispersed particles of athermosetting resin, an iron-tin alloy layer, an oxide layer and amagnetite layer, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2 showing the main parts of embodiments of theside seam-coated welded can according to the present invention, ametallic can blank 1 cut in a predetermined size is formed into acylinder, and the edges are lapped and welded to form a seam 2. Theinner face of the so formed can body, except the seam 2 or its adjoiningportion, may be coated with a protecting resin layer 10.

A cut edge 3 of the blank and a protrusion 4 formed by a pressureapplied to the melt of the metal blank at the welding step are presenton the seam 2 located on the inner face side of the can body. A resinlayer 5 is formed on the seam 2 so as to cover the cut edge 3 andprotrusion 4.

One of the important features of the present invention is that thisresin layer 5 is formed of a thermosetting resin and a thermoplasticresin at the above-mentioned specific volume ratio and one of the tworesins is present in the form of a continuous phase and at least a partof the other resin is present in the form of fine particles dispersed inthis continuous phase. In the present invention, the kind of the resinconstituting the continuous phase is determined by the volume ratio ofboth the resins. More specifically, when the thermosetting resin and thethermoplastic resin are present at a volume ratio (A) ranging from 95/5to 25/75, especially from 93/7 to 30/70, the thermosetting resin formsthe continuous phase and the thermoplastic resin forms the dispersedphase. When the thermosetting resin and the thermoplastic resin arepresent at a volume ratio (B) ranging from 1/99 to 20/80, especially,from 2/98 to 15/85, the thermoplastic resin forms the continuous phaseand at least a part of the thermosetting resin forms the dispersedphase.

FIG. 1 illustrates an embodiment of the dispersion state formed by thevolume ratio (A). More specifically, the resin coating layer 5 comprisesa continuous phase 6 composed of a thermosetting resin and a dispersedphase 7 composed of thermoplastic resin particles having a numberaverage particle size of 0.1 to 80 μm.

FIG. 2 illustrates an embodiment of the dispersion state formed by thevolume ratio (B). More specifically, the thermoplastic resin is presentin the form of a continuous phase 8 in the coating layer, a part of thethermosetting resin is present in the form of a continuous very thinlayer 9 in the interface between the continuous phase 8 of thethermoplastic resin and the inner face side of the seam, and theremaining part of the thermosetting resin is present in the form of finedispersed particles 11 in the continuous phase 8.

As pointed out hereinbefore, thermosetting resins have excellentadhesion to a metal and barrier property to corrosive components but itis poor in processability, and thermoplastic resins have excellentprocessability but poor adhesion and barrier properties.

Furthermore, when a solution of a thermosetting resin alone is coated onthe side seam, the solution flows to the stepped portion such as the cutedge 3 or protrusion 4, and as pointed out hereinbefore, the coatingbreaks in the angular portion 12 of the cut edge 3 or the thickness ofthe coating is reduced in this angular portion 12.

In contrast, when the thermosetting resin solution is used as thedispersion medium and the thermoplastic resin particles are used as thedispersed phase and when the volume ratio of the thermosetting resin andthe thermoplastic resin is controlled within the above-mentioned rangeaccording to the present invention, by virtue of thixotropiccharacteristics of this paint, the paint can be thickly applied even tothe angular portion of the seam 2 and the coating and baking operationscan be performed in this thickly applied state. Furthermore, in theportion where the paint is thus thickly coated, the thermoplastic resinparticles effectively retain the thermosetting resin solution on thesurfaces thereof, and this solution flows to the vacant space of thestepped portion to control the levelling tendency of the coating.

Accordingly, if the above-mentioned paint is used according to thepresent invention, even in the angular portion 12 of the cut edge, thethickness of the coating is substantially the same as in other portions,and the cut edge 3 can be coated entirely.

In the above-mentioned embodiment (A), the continuous phase 6 (seeFIG. 1) of the thermosetting resin is maintained until completion of thebaking operation, and the dispersed particles of the thermoplastic resinare softened or molten under baking conditions and complete adhesion isproduced between the thermosetting resin phase and the thermoplasticresin phase, whereby the mechanical strength, the impact resistance andthe processability can be remarkably improved. Furthermore, by thiscomplete adhesion between the two resin phases, the barrier property tocorrosive components can be more prominently improved.

According to this preferred embodiment of the present invention, byincluding the thermosetting resin in the form of a continuous phase inthe coating, the adhesion of the coating to the side seam and thebarrier property to corrosive components such as water, acids, oxygenand hydrogen sulfide can be remarkably improved, and at the same time,by including thermoplastic resin particles in the form of a dispersedphase in the coating, an excellent processability sufficient to resistthe double-seaming or bead-processing operation can be imparted to thecoated side seam.

In the above-mentioned embodiment (B), with evaporation of the solvent,a part of the thermosetting resin is deposited in the form of acontinuous very thin layer on the metal face of the seam and theremaining part of the thermosetting resin is deposited on the surfacesof the thermoplastic resin particles. In this state, baking of thecoating is further advanced to melt the thermoplastic resin particles.By controlling the volume ratio of both the resins within theabove-mentioned range (B), the thermoplastic resin is formed into acontinuous phase and the thermosetting resin left on the surfaces of theparticles is formed into a fine dispersed phase. The phase reversion isthus caused, and finally, curing of the thermosetting resin iscompleted.

According to this latter-mentioned embodiment, since a part of thethermosetting resin is present in the form of a thin layer adheringclosely to the entire metal face of the side seam, the adhesion of thecoating and the corrosion resistance can be remarkably improved.Furthermore, the thickness of this thermosetting resin thin layer iscontrolled within the range of 0.01 to 5 μm, and a continuous phase ofthe thermoplastic resin layer is formed on this thin layer. Accordingly,processability is remarkably improved, and even if the side seam-coatedwelded can is subjected to such a severe treatment as necked-intreatment, dissolution of the metal from the side seam or corrosion isadvantageously controlled.

In order to attain the objects of the present invention, it is importantthat the thermoplastic resin particles used for the paint of the presentinvention should have a softening point of 50° to 300° C., especially120° to 270° C., as determined according to the ring and ball method. Ifthe softening point of the thermoplastic resin is lower than 50° C.,both the resins are formed into a homogeneous mixture and the specificdispersion state of the present invention can hardly be produced, withthe result that the cut edge-coverage characteristic, the processabilityand the corrosion resistance are drastically reduced. On the other hand,if the softening point of the thermoplastic resin is higher than 300°C., the processability is much lower than the processability attainedwhen the softening point is within the range specified in the presentinvention, and furthermore, mutual bonding of both the resins in theinterface becomes difficult, resulting in reduction of the barrierproperty.

It is preferred that the number average particle size of thethermoplastic resin particles be 0.1 to 80 μm, especially 0.5 to 50 μm.If the number average particle size is either smaller than 0.1 μm orlarger than 80 μm, an optimum combination of the thick coating propertyand the control of flow of the thermosetting resin solution cannot beobtained. Especially in the above-mentioned embodiment (A), if thenumber average particle size is smaller than 0.1 μm, since the grainarea per unit volume of the coating becomes too large, corrosion such assulfide blackening by the content is easily advanced because ofreduction of the barrier property, and also the processability of thecoating after curing is degraded. If the number average particle size islarger than 80 μm, it is difficult to form uniformly in the coating adispersion texture comprising a continuous phase of the thermosettingresin and a dispersed phase of the thermoplastic resin, and the effectof completely coating the cut edge portion, the processability and thecorrosion resistance tend to drastically decrease.

This tendency is observed also in the embodiment (B) where the phaseinversion is caused between the thermoplastic resin particles and thethermosetting resin solution.

In the paint used in the present invention, if the volume ratio of thethermosetting resin and the thermoplastic resin is intermediate betweenthe above range (A) and the above range (B), it is very difficult toform a continuous phase of one of the two resins, and therefore,complete coating of the cut edge becomes very difficult and theprocessability is drastically reduced. If the volume ratio of thethermoplastic resin is lower than the lower limit of the above range,complete coating of the cut edge becomes difficult and prevention ofcorrosion of the cut edge portion becomes very difficult. Moreover, theprocessability of the resulting coating is degraded. If the volume ratioof the thermosetting resin is lower than the lower limit of the aboverange, the adhesion of the coating to the side seam and the corrosionresistance of the cut edge are reduced.

In order to improve the interfacial adhesion between both the resins, itis preferred that a thermoplastic polymer containing in the main chainor side chain a carboxyl group derived from a carboxylic acid, acarboxylic acid salt, a carboxylic acid anhydride, a carboxylic acidester, a carboxylic acid amide, a carbonic acid ester, urea or urethanecan be used as the thermoplastic resin. When a thermoplastic polymercontaining carboxyl groups at a concentration of 12 to 1400 meq,especially 50 to 1200 meq, per 100 g of the polymer is employed, bestresults are obtained with respect to the processability and thecorrosion resistance.

This thermoplastic polymer can be obtained by including a monomer havinga functional group such as mentioned above into the main chain bypolymerization or copolymerization or by coupling such monomer to thethermoplastic polymer by a grafting or terminal treatment. Furthermore,in case of a hydrocarbon type polymer such as an olefin resin, athermoplastic resin having carbonyl groups at the above-mentionedconcentration can be formed by subjecting this resin to an oxidizingtreatment.

Appropriate examples of such thermoplastic polymers are described below,though thermoplastic polymers that can be used in the present inventionare not limited to those described below.

(a) Polyesters comprising recurring units represented by the followinggeneral formula: ##STR1## wherein R₁ stands for an alkylene group having2 to 6 carbon atoms and R₂ stands for an alkylene or arylene grouphaving 2 to 24 carbon atoms.

For example, there can be mentioned polyethylene adipate, polyethylenesebacate, polyethylene terephthalate, polytetramethylene isophthalate,polyethylene terephthalate/isophthalate, polytetramethyleneterephthalate, polyethylene/tetramethylene terephthalate andpolyethylene/hydroxybenzoate.

(b) Homopolymers or copolymers of monomers represented by the followingformula: ##STR2## wherein R₃ stands for a hydrogen atom or a lower alkylgroup and R₄ stands for a hydrogen atom or an alkyl group having 1 to 12carbon atoms, and copolymers of monomers of the formula (3) with olefinsor other vinyl monomers, and acryl-modified polyolefins.

For example, there can be mentioned polyacrylic acid esters,polymethacrylic acid esters, ethylene/acrylic acid ester copolymers,acrylic acid ester/acrylic acid copolymers, ethylene/acrylic acidester/acrylic acid copolymers, ethylene/acrylic acid copolymers,styrene/methacrylic acid ester/acrylic acid copolymers, acrylic acidester/vinyl chloride copolymers, acrylic acid ester-graftedpolyethylene, methacrylic acid ester/vinyl chloride copolymers,styrene/methacrylic acid ester/butadiene copolymers and methacrylic acidester/acrylonitrile copolymers.

(c) Copolymers of vinyl esters represented by the following generalformula: ##STR3## wherein R₅ stands for a hydrogen atom, an alkyl groupor a phenyl group, with olefins or other vinyl monomers, and partiallysaponified products thereof.

For example, there can be mentioned partially saponified ethylene/vinylacetate copolymers, ethylene/vinyl propionate copolymers, ethylene/vinylacetate copolymers, acrylic acid ester/vinyl acetate copolymers andvinyl chloride/vinyl acetate copolymers.

(d) Ionomers, that is, resins obtained by neutralizing copolymers ofolefins with unsaturated carboxylic acids, optionally with other vinylmonomers, with an alkali metal, an alkaline earth metal or an organicbase.

For example, there can be mentioned Surlyns manufactured and marketed byDu Pont Co., U.S.A.

(e) Copolymers of maleic anhydride with other vinyl monomers and maleicanhydride-modified polyolefins.

For example, there can be mentioned maleic anhydride/styrene copolymers,maleic anhydride-modified polypropylene and maleic anhydride-modifiedpolyethylene.

(f) Polycarbonates having recurring units represented by the followingformula: ##STR4## wherein R₆ represents a hydrocarbon group having 8 to15 carbon atoms.

For example, there can be mentioned poly-p-xylene glycol biscarbonate,poly-dihydroxydiphenylmethane carbonate, polydihydroxydiphenylethanecarbonate, poly-dihydroxydiphenyl-2,2-propane carbonate andpoly-dihydroxydiphenyl-1,1-ethane carbonate.

(g) Polyamides having recurring units represented by the followinggeneral formula: ##STR5## wherein n is a number of from 3 to 13 and m isa number of from 4 to 11.

For Example, there can be mentioned poly-ω-aminocaproic acid,poly-ω-aminoheptanoic acid, poly-ω-aminocaprylic acid,poly-ω-aminopelargonic acid, poly-ω-aminodecanoic acid,poly-ω-aminoundecanoic acid, poly-ω-aminotridecanoic acid,polyhexamethylene adipamide, polyhexamethylene sabacamide,polyhexamethylene dodecamide, polyhexamethylene tridecamide,polydecamethylene adipamide, polydecamethylene sebacamide,polydecamethylene dodecamide, polydecamethylene tridecamide,polydodecamethylene adipamide, polydodecamethylene sebacamide,polydodecamethylene dodecamide, polydodecamethylene tridecamide,polytridecamethylene adipamide, polytridecamethylene sebacamide,polytridecamethylene dodecamide, polytridecamethylene tridecamide,polyhexamethylene azelamide, polydodecamethylene azelamide andpolytridecamethylene azelamide.

(h) Polyureas having recurring units represented by the followinggeneral formula: ##STR6## wherein R₇ and R₈ stand for an alkylene grouphaving 1 to 13 carbon atoms.

For example, there can be mentioned polyhexamethyleneurea,polyheptamethylene-urea, polyundecamethylene-urea andpolynonamethylene-urea.

(i) Polyurethanes and polyureaurethanes represented by the followinggeneral formula: ##STR7## wherein R₉ stands for an alkylene group having3 to 24 carbon atoms or a polyether or polyester residue having 3 to 24carbon atoms, R₁₀ stands for an alkylene or arylene group having 3 to 24carbon atoms, R₁₁ stands for an alkylene or arylene group having 1 to 13carbon atoms, and k is a number of 0 or 1.

For example, there can be mentionedpolytetramethylenehexamethylene-urea,polyhexamethylene-tetramethyleneurea, and polyureaurethanes formed bychain-extending isocyanate-terminated polyesters or polyethers with adiamine or water.

(j) Resin particles obtained by oxidizing particles of polyethylene,polypropylene or a crystalline ethylene/propylene copolymer with oxygen,ozone or other oxidant.

As the thermoplastic resins preferred for attaining the objects of thepresent invention, there can be mentioned, in an order of importance,polyesters, polycarbonates, polyamides, ionomers and acid-modifiedpolyolefins.

These resins should have at least a film-forming molecular weight. Knownadditives such as ultraviolet absorbents, stabilizers, lubricants,antioxidants, pigments, dyes and antistatic agents may be incorporatedin these resin particles according to known recipes, if desired.

Optional means can be adopted for pulverizing the thermoplastic resininto particles having the above-mentioned particle size. For example, incase of addition polymers, resin particles having a predeterminedparticle size can be obtained by emulsion-polymerizing constituentmonomers. Furthermore, there may be adopted a method in which the resinis pulverized under cooling, a method in which the resin is dissolved ata high temperature and the solution is cooled to precipitate the resinin the form of particles, a method in which a resin solution is broughtinto contact with a non-solvent to coagulate and precipitate the resinin the form of particles, and a method in which a resin solution issprayed into a gas current to precipitate the resin in the form ofparticles. The resin particles prepared according to these methods maybe sieved and classified to obtain particles having a predeterminedparticle size, if necessary.

Any of thermosetting resins customarily used in the field of themanufacture of paints can be used as the thermosetting resin in thepresent invention. As preferred thermosetting resins, there can bementioned, for example, phenol-formaldehyde resins, furan-formaldehyderesins, xylene-formaldehyde resins, ketone-formaldehyde resins,urea-formaldehyde resins, melamine-formaldehyde resins, alkyd resins,unsaturated polyester resins, epoxy resins, bismaleimide resins,triallyl cyanurate resins, thermosetting acrylic resins, silicone resinsand oleoresins. These resins may be used singly or in the form of amixture of two or more of them.

From the viewpoints of adhesion to the side seam and corrosionresistance, it is preferred that a coating-forming resin comprising anepoxy resin component and at least one thermosetting resin selected fromthe group consisting of phenolic resins, urea resins, melamine resinsand thermosetting acrylic resins can be used as the thermosetting resin.This coating-forming resin may be used in the form of a mixture orprecondensate of the constituent resins.

The paint used in the present invention can be easily prepared bydissolving the above-mentioned thermosetting resin in an appropriateorganic solvent and dispersing thermoplastic resin particles in thesolution. A solvent capable of dissolving the thermosetting resin butincapable of dissolving the thermoplastic resin is chosen as thesolvent. Ordinarily, there are used aromatic solvents such as xylene andtoluene, ketone type solvents such as acetone, methylethyl ketone andcyclohexanone, alcohol type solvents such as ethanol and butanol, cyclicand linear ethers such as tetrahydrofuran, dioxane and cellosolves, andesters such as ethyl acetate and butyl acetate. These solvents may beused singly or in the form of a mixture of two or more of them. Thethermoplastic resin particles may be added in the form of a dispersionto the thermosetting resin solution.

In order to obtain a thick coating completely covering the cut edgeportion, it is preferred that when the paint used in the presentinvention is dropped on a glass sheet in an amount of 0.2 mm³ at 25° C.and allowed to stand for 1 minute, the contact angle be 18.5° to 45°,especially 19.0° to 43°. The solid concentration of the paint isappropriately chosen within the range of 5 to 50% according to thecombination of the resins so that a good combination of the adaptabilityto the coating operation and the thick coating property can be obtained.

As the material constituting the can body, there can be mentioned, forexample, an untreated steel plate (black plate), electrically plated andmelt-plated steel plates such as a tinplate, a zinc-plated steel plateand a chromium-plated steel plate, steel plates chemically treated withchromic acid, phosphoric acid and the like, and steel plateselectrolytically treated with chromic acid. Furthermore, a light metalsheet such as an aluminum sheet can be used.

The side seam can be advantageously formed by electric resistancewelding. Formation of a side seam by electric resistance welding isaccomplished by forming a can blank into a cylinder and passing thelapped edges through between a pair of electrode rollers or passing thelapped edges between a pair of upper and lower electrodes through anelectrode wire. In order to prevent formation of porous metal oxides onthe surface of the side seam and improve the adhesion of the protectingpaint, it is preferred that the welding operation be carried out in aninert atmosphere and this inert atmosphere be maintained until thesurface temperature of the welded portion is lowered to 550° C. As theinert atmosphere, there can be used, for example, nitrogen, argon, neon,hydrogen and carbon dioxide. It is ordinarily preferred that the weldingoperation be carried out while holding the portion to be welded in acurrent of an inert gas such as mentioned above, but the weldingoperation may be carried out in a sealed vessel filled with the inertgas.

In accordance with another preferred embodiment of the presentinvention, the above-mentioned side seam-coating paint is applied to acan body having a side seam formed by welding of a tinplate, the innerand outer surface portions of said side seam comprising an outermostlayer composed of an oxide, which has a thickness of 50 to 400 Å, and asubsequent intermediate iron-tin alloy layer having a thicknesscorresponding to 5 to 100% of the thickness of the total tin layer inthe starting tinplate.

Referring to FIG. 3 illustrating in an enlarged manner the sectionalstructure of the surface portion of this welded tinplate can, aniron-tin alloy layer 14 is formed on a steel substrate 13 and an oxidelayer 15 is formed on the alloy layer 14. One of the characteristicfeatures of this seam is that the thickness of the oxide layer 15 isrestricted to 50 to 400 Å and the thickness of the iron-tin alloy layercorresponds to 5 to 100%, especially 15 to 100%, of the thickness of thetotal tin layer in the starting tinplate can blank 1.

In ordinary tinplate can blanks, the thickness of the total tin layer isabout 0.38 to about 1.52 μm, and an iron-tin alloy layer is inevitablypresent between the metallic tin layer and the steel substrate. Thethickness of this iron-tin alloy layer is relatively small andcorresponds to 5 to 30% of the thickness of the total tin layer, thoughthe thickness differs to some extent according to thetinplate-manufacturing process. In conventional welded cans, themetallic tin layer is substantially removed on the surface of the weldedportion for the reasons described hereinbefore, and instead of theremoved metallic tin layer, there is formed an iron oxide layer having athickness of 700 to 1000 Å.

In contrast, in the welded can of the above-mentioned type according tothe present invention, the thickness of the iron-tin alloy layer isincreased to a level corresponding to 5 to 100% of the thickness of thetotal tin layer and the thickness of the oxide layer is controlled tothe above-mentioned low level of 50 to 400 Å. In the welded can of thistype according to the present invention, since the thickness of theiron-tin alloy layer having a fine and compact texture is increased, thecorrosion resistance of the bonded portion is remarkably increased andthe processability of the surface of the bonded portion and the adhesionof the resin coating to the surface of the bonded portion are highlyimproved. Furthermore, since the oxide layer which is very thin andhence, has no bad influence on the processability is formed on thisalloy layer, the adhesion of the resin coating to the side seam isremarkably enhanced, with the result that even if the welded can issubjected to a severe processing treatment or hydrothermal treatment(for example, the retort sterilization treatment), a high adhesion ofthe side seam to the resin coating 5 and a high corrosion resistance ofthe side seam can be maintained.

The thickness of the iron-tin alloy layer 14 can be measured by theX-ray fluorometry. This thickness can be measured also by a scanningtype electron microscope or an X-ray microanalyzer.

In the present embodiment, the composition of the iron-tin alloy layer14 is not particularly critical, but the Fe/Sn atomic ratio isordinarily in the range of from 1/1 to 1/2. The oxide layer 15 consistsmainly of a continuous layer 16 of iron oxide, but a tin oxide layer 17or a metallic tin layer 18 may be present in the form of islands orcontinuous layers.

If the thickness of the iron-tin alloy layer is too small and below theabove-mentioned range, no satisfactory results can be obtained withrespect to the adhesion of the coating, the corrosion resistance and theprocessability. If the thickness of the oxide layer is too large andexceeds the above-mentioned range, no satisfactory results can beobtained with respect to the above characteristics. Furthermore, if thethickness of the oxide layer exceeds the above-mentioned range, also theappearance characteristic is degraded with reduction of the adhesion ofthe paint.

In the side seam-coated welded can of the present invention, the widthof the side seam may be relatively small and is ordinarily in the rangeof 0.2 to 1.2 mm, though the width of the side seam differs to someextent according to the can diameter. Accordingly, the amount used ofthe can blank can be reduced. This is another advantage attainedaccording to the present invention. As the tinplate blank, there can beused either a tin melt-plated steel plate or an electrically tin-platedsteel plate. Either a reflow tinplate or a non-reflow tinplate can beused as the electrically tin-plated steel plate. The amount deposited oftin is preferably in the range of 1.12 to 11.2 g/m², and the thicknessof the tinplate is preferably in the range of from 0.15 to 0.50 mm.

In order to form the welded side seam defined according to the presentinvention, it is important that the above-mentioned welding means beadopted and that the rate of lowering the surface temperature of theside seam from the welding temperature (which is in the range of fromabout 750° to about 1450° C.) to 550° C. should be in the range of 100°to 900° C./sec. This cooling rate has close relations to the thicknessof the iron-tin alloy layer and the thickness of the oxide layer. Whenthe cooling rate is higher than 900° C./sec, the thickness of the alloylayer is smaller than the value specified in the present invention, andif the cooling rate is lower than 100° C./sec, the thickness of theoxide layer tends to exceed the range defined in the present invention.If the cooling rate is controlled within the above-mentioned range, itis possible to form tin-iron alloy and oxide layers having thicknesscharacteristics most preferred for various characteristics.

In accordance with still another preferred embodiment of the presentinvention, the above-mentioned paint is applied to a side seam weldedcan formed by welding the side portions of a can blank (tin-free steelcan blank) comprising a steel plate substrate, a metallic chromium layerformed on the substrate and a chromium oxide layer formed on themetallic chromium layer to form a side seam having a steelsubstrate-exposed layer which has a thickness of 50 to 800 Å and iscomposed of a fine and compact steel oxide consisting mainly ofmagnetite.

Referring to FIG. 4 illustrating in an enlarged manner the side seam ofthis welded can and the adjoining portion thereof, the tin-free steelblank 1 (hereinafter referred to as "TFS blank") comprises a steel platesubstrate 13, a metallic chromium layer 19 and a chromium oxide surfacelayer 20. In the area of the side seam 3, these chromium-containinglayers are removed and the steel substrate 13 is exposed to form a steeloxide layer 21.

The steel substrate 13 in the TFS blank is formed, for example, by coldrolling of low-carbon steel, and the thickness of the metallic chromiumlayer 19 or chromium oxide layer 20 or the thickness ratio of the twolayers considerably differs according to the intended use of the weldedcan or the volume of the can. Generally, the metallic chromium layer hasan amount deposited of 10 to 100 mg/m², especially 20 to 80 mg/m², andthe chromium oxide layer has an amount deposited of 0.5 to 20 mg/m²,especially 1 to 10 mg/m².

One of the important features of this TFS can is that theabove-mentioned steel oxide layer 21 is formed of a fine and compactsteel oxide consisting mainly of magnetite (Fe₃ O₄) and the thickness ofthis layer is controlled to 50 to 300 Å, especially 50 to 400 Å.

In the conventional welded TFS can, on the steel substrate exposedsurface to be formed into a side seam, a steel oxide layer having athickness of 1000 to 3000 Å is formed, and this oxide layer is veryporous and in the chemical composition of this oxide layer, the ratio ofthe oxygen atoms to the Fe atoms is relatively high. This oxide layer isreadily influenced by the atmosphere. For example, if this oxide layeris brought into contact with air for a relatively short time, it isreadily converted to hydrous ferric oxide (so-called red rust).

In contrast, in the welded can of the above-mentioned type according tothe present invention, since the steel oxide layer is formed of a fineand compact oxide consisting mainly of magnetite and the thickness ofthis layer is controlled to a very small level of 50 to 800 Å, theadhesion of the coating and the corrosion resistance are remarkablyimproved, and these excellent characteristics of the seam can bemaintained even after processing. For example, when the thickness ofthis magnetite layer is smaller than 50 Å, the adhesion of the coatingis reduced, and if the thickness of the magnetite layer is larger than800 Å, the adhesion of the coating and the corrosion resistance afterprocessing are degraded. Moreover, various advantages are attained withrespect to the anti-corrosive characteristic by the feature that theoxide layer is formed of a fine and compact oxide consisting mainly ofmagnetite.

The fact that the steel oxide layer is formed of magnetite can beconfirmed by X-ray diffractiometry or electron beam diffractiometry. Thechemical composition of the oxide layer can be determined by the ESCAmethod (electron spectroscopy for chemical analysis) in which thesurface is etched with Ar gas and the atomic concentration ratio of Oand Fe is measured. The thickness of this steel oxide layer can bemeasured by a scanning type electron microscope or an X-raymicroanalyzer.

This welded TFS can is formed by forming a TFS can blank into a cylinderand passing the lapped edges through a pair of electrode rollers orpassing the lapped edges between a pair of upper and lower electroderollers through an electrode wire. It is important that this weldingoperation should be carried out in an inert atmosphere and this inertatmosphere should be maintained until the surface temperature of thewelded portion is lowered to 550° C. Two excellent effects of reducingthe thickness of the oxide layer and converting the steel oxide tomagnetite can be attained by adoption of an inert atmosphere, that is, anon-oxidizing atmosphere, either as the welding atmosphere or as theatmosphere of the subsequent gradually cooling zone.

Application of the paint to the side seam of the can body and theadjoining portion thereof is performed by known means such as rollercoating, spray coating, brush coating, flow coating and dip coating, andspecial means such as powder coating need not be adopted and existingcoating equipment can be used without any particular modification. Thisis still another prominent advantage that can be attained according tothe present invention.

In order to completely coat even the angular portion of the cut edge ofthe side seam, it is preferred that the thickness of the coating in theangular portion of the cut edge after baking be 2 to 50 μm. When theabove-mentioned paint of the present invention is used, such coating caneasily be formed.

According to the present invention, the so formed coating is heated at atemperature higher than the softening point or melting point of thethermoplastic resin to effect curing of the coating. As heating means,there can be employed direct flame heating, hot air oven heating,induction heating, resistance heating and infrared ray heating. Thecuring conditions are appropriately chosen from curing temperatures of150° to 400° C. and curing times of 1 second to 20 minutes so that anet-like structure is formed in the thermosetting resin by cross-linkingand a sufficient bonding is formed between the thermosetting resin andthe thermoplastic resin in the grain boundary.

From the viewpoint of the mechanical strength of the coating, it ispreferred that the thermoplastic resin used be crystalline. In thiscase, in order to improve the processability and the barrier property(the corrosion resistance by good adhesion in the grain boundary), it ispreferred that the temperature of the coating be rapidly lowered fromthe baking temperature to a level lower by at least 10° C. than thecrystallizing temperature of the thermoplastic resin within 10 seconds,especially 1 second, so as to inhibit or control crystallization of thethermoplastic resin in the coating. This rapid cooling can easily beaccomplished by bringing the baked coating into contact with coolingwater, a liquefied nitrogen current, cold air or other cooling medium orby bringing the side seam of the baked coating into contact with acooling apparatus such as a chilling roller.

The side seam-coated welded can of the present invention can be used invarious fields as a vacuum can which is subjected to retortsterilization of the content, an inner pressure can in which acarbonated drink or the like is packed or an aerosol can.

The present invention will now be described in detail with reference tothe following Examples that by no means limit the scope of theinvention.

Thermosetting resin solutions used in the Examples were preparedaccording to the methods described below.

(1) Epoxy-Urea Type Resin Solution:

70 Parts of a condensation product of bisphenol A and epichlorohydrin,which had an average molecular weight of 2900, and 30 parts of a butylether-urea-formaldehyde resin were dissolved in (I) a mixed solventcomprising 50 parts of dioxane, 15 parts of xylene, 15 parts ofcyclohexanone, 10 parts of methylethyl ketone and 10 parts of toluene or(II) a mixed solvent comprising 50 parts of methylethyl ketone and 50parts of toluene to form a solution (I) or (II) having a solid contentof 25%. Solution (I) was used as the base paint in sample number 24 inExample 4, and solution (II) was used as the base paint in samplenumbers 20 to 23 and 25 in Examples 4 and 6 respectively.

(2) Epoxy-Phenol Type Resin Solution:

In 1.5 mols of a 37% aqueous solution of formaldehyde were dissolved 0.5mol carbolic acid and 0.5 mol of p-cresol, and 0.15 mol of ammonia wasadded as a catalyst and reaction was carried out at 95° C. for 3 hours.The reaction product was extracted with a mixed solvent comprisingketone, alcohol and hydrocarbon, and the extract was washed with waterand the water layer was removed. Residual water was removed byazeotropic distillation, and the residue was cooled to form a 30%solution of a resol type phenolic resin. This resol type phenolic resinsolution was mixed with a 30% solution of an epoxy resin having amolecular weight of 2900, which was a condensation product of bisphenolA and epichlorohydrin, in a mixed solvent comprising ketone, ester,alcohol and hydrocarbon. The weight ratio of the phenolic resin to theepoxy resin was 40/60. This mixture was pre-condensed under reflux for 2hours to form an epoxy-phenol resin solution. This solution was used asthe base paint in Examples 1, 2, 3 and 5.

Welded can bodies used in the Examples were prepared according to thefollowing procedures.

In case of welded tinplate cans used in Examples 3 and 5, anepoxy-phenol type paint (a 1/1 mixture of an epoxy resin and a phenolicresin) was margin-coated on a tinplate having a thickness of 0.21 mm, ahardness of T-4, a plated tin amount of 50 lb/B.B and a tin layerthickness of 1.2 μm, except the area of the side seam of the can body,so that the thickness of the coating after baking was 5 μm on the innerface side and 3 μm on the outer face side, and the applied paint wasbaked and cured for 10 minutes in a hot air drying furnace maintained at200° C. The coated tinplate was cut into a body blank of can having alength of 206.4 mm and a height of 100.5 mm, and the blank is formedinto a cylinder by a roll former so that the short side was located inthe axial direction. In the welding station, the cut edges were lappedand fixed, and by using a commercially available seam welding machinecomprising two roll electrodes disposed through a wire electrode and aninert gas feed nozzle, a pressing force of 40 Kg/mm² was applied to thelapped portion of the formed cylinder and a welded can having a diameterof 2×11/16 inches and an inner volume of 318.2 ml at a can manufacturingspeed of 30 m/min in a nitrogen gas current supplied at a rate of 5l/min. The lap width before welding was 0.3 mm and the lap width afterwelding was 0.4 mm.

In Example 1, welded cans prepared in the same manner as described abovein a nitrogen gas current or air current supplied at a flow rate shownbelow were used.

In case of welded TFS welded cans, an epoxy-phenol type resin (1/1mixture of an epoxy resin and a phenolic resin) was margin-coated on aTFS plate having a thickness of 0.17 mm and a hardness of T-4 except thearea of the side seam of the can body so that the thickness of thecoating after baking was 5 μm on the inner face side and 3 μm on theouter face side, and the coated paint was baked and cured for 10 minutesin a hot air drying furnace maintained at 200° C. The coated TFS platewas cut into a body blank of can having a length of 206.4 mm and aheight of 104.5 mm. Both surfaces of the portion 6 near the cut edges,which were to be lapped together, were subjected to edge cleaning by thecutting method along the width of about 1 mm from each cut edge, wherebythe surface chromium layer and chromium oxide layer were removed. Theedge-cleaned blank was formed into a cylinder so that the short side waslocated in the axial direction. The cut edges were lapped and fixed inthe welding station, and by using a commercially available seam weldingmachine comprising two roll electrodes disposed through a wire electrodeand an inert gas supply nozzle, a pressing force of 40 Kg/m² was appliedto the lapped cut edges of the cylinder and a welded can body having adiameter of 2×11/16 inches and an inner volume of 318.2 ml at a canmanufacturing speed of 30 m/min in a current of a mixed inert gas (90mol % of nitrogen and 10 mol % of hydrogen) supplied at a rate of 5l/min. The lap width before welding was 0.3 mm and the lap width afterwelding was 0.4 mm.

In Example 2, welded TFS can bodies prepared in the same manner asdescribed above in a mixed gas current or air current supplied at a flowrate described hereinafter were used.

In the Examples, the properties of cans, the physical properties ofpaints and the structural and physical properties of the coatings nearcan side seams were determined and evaluated according to the followingmethods, and the surface metal layers of the welded portions wereanalyzed according to the methods described below.

(1) Contact Angle:

A micro slide glass sheet (grade 1: thickness=0.9-1.2 mm) was used asthe glass sheet, and the glass sheet was dipped in mixed chromic acidovernight and washed with water sufficiently. The glass sheet was driedand stored in the absolutely dry condition in a desiccator.

The measurement was carried out in a thermostat chamber maintained at25° C. and in an air atmosphere, 0.2 mm³ of the sample paint was droppedon the glass sheet by a micro syringe. After passing of 1 minute, thecontact angle was directly read from a goniometer.

(2) Analysis of Surface Metal Layers of Welded Portion:

(a) Thickness of Surface Oxide Layer:

Only the welded lap portions were cut out from the welded can, and thecut pieces were connected by an adhesive tape to form a sample having asize of 8 mm×8 mm. The surface of the sample was etched with Ar gas andthe atomic concentration ratio of O, Fe and Sn (in case of weldedtinplate cans) was measured according to the ESCA method. Since oxygenin the oxide was etched at a rate of 15 Å/min and the atomicconcentration ratio of oxygen was substantially kept constant at 10 to20% while etching was continued, the thickness of the portion where theatomic concentration ratio was higher than 20% was measured and thisthickness was defined as the thickness of the oxide layer.

(b) Thickness of Alloy Layer (in case of welded tinplate cans):

Only the welded lap portions were cut out from the welded can and thecut pieces were connected by an adhesive tape to form a sample having asize of 15 mm×15 mm. The sample was electrolytically treated in an NaOHsolution to remove free Sn from the surface of the welded portion. Then,the amount of Sn in the alloy layer was determined according to theX-ray fluorometry, and the thickness of the alloy layer was calculatedbased on the assumption that 1 g/m² of the amount of Sn in the alloylayer corresponded to the thickness of 0.144 μm. In case of free Sn, thethickness was calculated based on the assumption that 1 g/m² of theamount of Sn corresponded to the thickness of 0.137 μm.

(3) Observation of Dispersion State:

The cured coating was peeled off from the side seam portion of the canbody, and a test piece having a width of 2 mm and a length of 15 mm wascut out in parallel to the side seam. This test piece was embedded andsolidified in an epoxy resin and a slide having a thickness of about 15μm was cut out by a stainless steel microtome so that a section verticalto the seam was obtained. The slide was dipped in an aqueous solution ofa blue dye (Methyl Violet) for 10 minutes at normal temperatures toselectively dye the thermosetting resin. The dyed slice was washed withwater sufficiently, and the structure, especially the dispersion state,was observed by an optical microscope. The observed structure wasphotographed and the average particle size of particles present in thevisual field at the time of observation was measured. The number averageparticle size was adopted as the average particle size. The numberaverage particle size will be referred to merely as "particle size" or"average particle size" hereinafter.

(4) Side Seam-Covering Property (current value at constant voltageelectrolysis):

The side seam of the can body formed by coating, baking and curing ofthe sample paint was cut out along a width of 2 cm, and a portion havinga width of 3 mm in a direction rectangular to the side seam and a lengthof 100 mm in a direction parallel to the side seam was removed and theremaining side seam was sealed by an adhesive vinyl tape to form a testpiece. This test piece was dipped for 3 minutes in an electrolyteconsisting of a 3% aqueous solution of sodium chloride and maintained at25° C. Then, the test piece was subjected to the constant voltageelectrolysis for 10 seconds under a voltage of 10.0 V by using a carbonrod as a counter electrode and the average value of the flowing currentwas measured. Five test pieces were tested with respect to one sampleand the arithmetic mean value was calculated.

(5) Processability:

A test piece having a width of 40 mm and a length of 60 mm with the sideseam being as the center was cut out from the side seamed can bodyformed by coating, baking and curing of the sample paint. The flexuralstrength test was carried out according to the bending resistance testmethod of JIS K-5400, 6-16.

From the test piece which had been subjected to the flexural strengthtest, a portion having a width of 3 mm with the side seam being thecenter and a length of 6 mm in parallel to the side seam with the benttop end being the center was removed, and the remaining portion wassealed by an adhesive vinyl tape and was subjected to constant voltageelectrolysis under the same conditions as described in (4) above. Fivetest pieces were tested with respect to one sample, and the arithmeticmean value was calculated.

(6) Adhesion:

Square cuts having a side of 1 mm were formed on the coating of thewelded side seam portion after baking and curing, and an adhesivecellophane tape was applied to the coating and the peel test was thencarried out. The test was made on five optionally chosen cans withrespect to one sample paint, and the obtained results were evaluated asfollows:

○: no peel observed in any can

Δ: peel observed in 1 or 2 cans

X: peel observed in 3 or more cans

(7) Necked-In Processability:

The presence or absence of defects such as peeling and cracking on thecoating of the side seam portion was checked after necked-in processing.The test was made on optionally chosen 50 cans with respect to onesample paint.

(8) Evaluation of Actually Packed Cans:

(a) Amount of formed hydrogen:

At the time of opening, the gas in the can was collected and the amountof hydrogen was determined according to gas chromatography. Ten canswere tested with respect to one sample, and the arithmetic mean valuewas calculated. When swollen cans were formed, this fact was indicated.

(b) Amount of iron dissolved out:

Cans packed with an apple drink or consomme were tested. After opening,all the content was reduced to ashes. The ashes were dissolved inhydrochloric acid and the supernatant was subjected to atomic absorptionspectroscopy to determine the amount of iron in the content. Optionallychosen 10 cans were tested with one sample, and the arithmetic meanvalue was calculated.

(c) Perforation and state of welded side seam portion on inner face ofcan:

Packed cans were stored at 37° C. for 1 year and leakage of the content(liquid) was checked by observation with the naked eye. Cans whereleakage of the content was observed were picked up and the portions nearthe side seams were observed by a microscope after opening. Cans havingpierced holes were designated as "perforated cans", and the ratio of thenumber of perforated cans to the total number of tested cans wascalculated. After opening, the corrosion state of the portion near theside seam was checked by observation with the naked eye or bymicroscopic observation. With respect to one sample paint, 100 cans weretested to the storage test, and the corrosion state was examined onoptionally chosen 50 cans.

EXAMPLE 1

A powder having an average particle size of about 10 μm, which wasobtained by mechanically pulverizing from the pellets of nylon 12 havinga softening point of 173° C. and a carbonyl group concentration of 508meq per 100 g of the polymer, was used as the thermoplastic resinpowder, and the above-mentioned epoxy-phenol type resin solution wasused as the thermosetting resin solution. By using the same diluentsolvent as used for the thermosetting resin solution, both resins weremixed by a high speed mixer to obtain a paint having a solid content ofabout 25%, in which the ratio of the thermoplastic resin to thethermosetting resin in the solids was 30/70.

In order to obtain welded tinplate can bodies differing in structure ofthe surface metal layer in the welded side seam portion, nitrogencurrent flow rates of 20 l/min (sample No. 1), 5 l/min (sample No. 2)and 2 l/min (sample No. 3) were adopted, respectively, for the weldingoperation. For comparison, the flow rate of the nitrogen current wasreduced to 0.2 l/min (sample No. 4), and the welding operation wascarried out in air (sample No. 5). Other welding conditions were thesame as described hereinbefore.

With respect to each of the so obtained can bodies, the above-mentionedpaint was spray-coated on the inner and outer surfaces of the side seamportion of the can body along a width of about 10 mm by an airless spraygun while maintaining the paint temperature at 40° to 70° C. during thespraying operation, so that the thickness of the coating after dryingwas 40 μm. Then, hot air was blown to the coated can from below forabout 10 seconds in the state where the side seam on the inner face ofthe can was located above, whereby the solvent was substantiallyevaporated. The coated paint was then baked and cured in a gas ovenmaintained at 220° C. for about 60 seconds to obtain a can body having acoated side seam.

The so obtained can body was tested with respect to items shown in Table1, and the surface metal layer of the welded portion of the welded canbody before the coating operation was analyzed.

The so obtained can body having a coated side seam was subjected to beadprocessing, necked-in processing and flange processing, and a tinplatelid for a can having an inner diameter of 2 9/16 inches, which had anepoxy-phenol coating on each of the inner and outer surfaces, wasdouble-seamed to the processed can body. An apple drink heated at 90° C.or tomato sauce was packed in the obtained empty can, and the sametinplate lid as described above was double-seamed to the packed can by avacuum seamer. The can packed with the tomato sauce was subjected toheat sterilization at 118° C. for 90 minutes. Each packed can was storedat 37° C. for 1 year.

With respect to each sample, 100 cans were prepared and tested. Theamount of iron dissolved out and the amount of formed hydrogen weredetermined with optionally chosen 10 cans, and 50 cans optionally chosenfrom the remaining cans were opened and the corrosion state of the sideseam portion of the can body was examined.

The obtained results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Results of Analysis of                                                        Surface Metal Layer of                                                        Welded Portion                                                                    thickness                                                                           ratio (%) of                                                            (Å) of                                                                          thickness of                                                                         Side Seam-         State of                                      surface                                                                             alloy layer                                                                          Covering                                                                              Processa-  Necked-in                                 Sample                                                                            oxide to total plat-                                                                       Property                                                                              bility Adhe-                                                                             Processed                                 No. layer ing thickness                                                                        (mA/300 mm.sup.2)                                                                     (mA/18 mm.sup.2)                                                                     sion                                                                              Portion                                   __________________________________________________________________________    Samples of Present Invention                                                  1   60-70  7-10  0       0      O   no change                                 2   150-170                                                                             60-71  0       0      O   no change                                 3   350-380                                                                             90-95  0       0      O   no change                                 Comparative Samples                                                           4   700-800                                                                              3-6   0       6.2    Δ                                                                           no change                                 5   1400-1600                                                                           15-23  0       1.0    X   peeling in                                                                    some cans                                 __________________________________________________________________________    Packed Can Test results                                                       apple drink                                                                   amount                 tomato sauce                                               (ppm) of           amount                                                     iron        number of                                                                            (ml/can)     number of                                 Sample                                                                            dissolved                                                                           state of                                                                            perforated                                                                           of formed                                                                           state of                                                                             perforated                                No. out   side seam                                                                           cans   hydrogen                                                                            side seam                                                                            cans                                      __________________________________________________________________________    Samples of Present Invention                                                  1   5.4   no change                                                                           0      0.13  no change                                                                            0                                         2   4.4   no change                                                                           0      0.12  no change                                                                            0                                         3   6.2   no change                                                                           0      0.19  no change                                                                            0                                         Comparative Samples                                                           4   9.2   blister of                                                                          0      2.11  blister in                                                                           1                                                   processed          processed                                                  portion in         portion                                                    some cans          red rust                                         5   13.3  blister in                                                                          0      some cans                                                                           blister in                                                                           8                                                   processed    swollen                                                                             processed                                                  portion            portion,                                                   portion            red rust                                         __________________________________________________________________________

EXAMPLE 2

The same side seam coating paint as used in Example 1 was used.

In order to obtain welded TFS can bodies differing in structure of thesurface metal layer in the welded portion the above-mentioned inert gasmixture current flow rates of 20 l/min (sample No. 6), 5 l/min (sampleNo. 7) and nitrogen current flow rates of 2 l/min (sample No. 8) wereadopted, respectively, for the welding operation. For comparison, theflow rate of the nitrogen current was reduced to 0.2 l/min (sample No.9), and the welding operation was carried out in air (sample No. 10).Other welding conditions were the same as described hereinbefore.Furthermore, sample No. 6 was subjected to acid washing (pickling) withan aqueous solution containing 70 g/l of sulfuric acid for 30 seconds(sample No. 11).

With respect to each of the so obtained can bodies, the above-mentionedpaint was spray-coated on the inner and outer surfaces of the side seamportion of the can body along a width of about 10 mm by an airless spraygun while maintaining the paint temperature at 40° to 70° C. during thespraying operation, so that the thickness of the coating after dryingwas 40 μm. Then, hot air was blown to the coated can from below forabout 10 seconds in the state where the side seam on the inner face ofthe can was located above, whereby the solvent was substantiallyevaporated. The coated paint was then baked and cured in a gas ovenmaintained at 220° C. for about 60 seconds to obtain a can body having acoated side seam.

The so obtained can body was tested with respect to items shown in Table2, and the surface metal layer of the welded portion of the welded canbody before the coating operation was analyzed.

The so obtained can body having a coated side seam was subjected to beadprocessing, necked-in processing and flange processing, and a TFS lidfor a can having an inner diameter of 2 9/16 inches, which had anepoxy-phenol coating on each of the inner and outer surfaces, wasdouble-seamed to the processed can body. An apple drink heated at 90° C.or tomato sauce was packed in the obtained empty can, and the same TFSlid as described above was double-seamed to the packed can by a vacuumseamer. The can packed with the tomato sauce was subjected to heatsterilization at 118° C. for 90 minutes. Each packed can was stored at37° C. for 1 year.

With respect to each sample, 100 cans were prepared and tested. Theamount of iron dissolved out and the amount of formed hydrogen weredetermined with optionally chosen 10 cans, and 50 cans optionally chosenfrom the remaining cans were opened and the corrosion state of the sideseam portion of the can body was examined.

The obtained results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________         Results of Analysis of                                                        Surface Metal Layer                                                                       Side Seam-                                                        of Welded Portion                                                                         Covering           State of                                       thickness (Å)                                                                         Property                                                                             Processa-   Necked-In                                 Sample                                                                             of magnetite                                                                              (mA/   bility      Processed                                 No.  layer       300 mm.sup.2)                                                                        (mA/18 mm.sup.2)                                                                     Adhesion                                                                           Portion                                   __________________________________________________________________________    Samples of Present Invention                                                  6     80-100     0      0      O    no change                                 7    250-280     0      0      O    no change                                 8    600-700     0      0      O    no change                                 Comparative Samples                                                           9    1300-1400   0      0.3    Δ                                                                            partially                                                                     peeled                                    10    200-2200   0      1.2    X    peeled                                    11   below 10    0      0.1    Δ                                                                            partially                                                                     peeled                                    __________________________________________________________________________    Packed Can Test Results                                                       apple drink                                                                   amount                 tomato sauce                                               (ppm) of           amount                                                     iron        number of                                                                            (ml/can)     number of                                 Sample                                                                            dissolved                                                                           state of                                                                            perforated                                                                           of formed                                                                           state of                                                                             perforated                                No. out   side seam                                                                           cans   hydrogen                                                                            side seam                                                                            cans                                      __________________________________________________________________________    Samples of Present Invention                                                  6   1.2   no change                                                                           0      0.06  no change                                                                            0                                         7   1.8   no change                                                                           0      0.08  no change                                                                            0                                         8   1.4   no change                                                                           0      0.07  no change                                                                            0                                         Comparative Samples                                                           9   4.2   blister in                                                                          0      0.51  blister in                                                                           O                                                   processed          processed                                                  portion in         portion in                                                 some cans          some cans,                                                                    red rust                                         10  17.3  blister in                                                                          0      some cans                                                                           blister in                                                                           12                                                  processed          processed                                                  portion in         portion in                                                 some cans          some cans                                        __________________________________________________________________________

EXAMPLE 3

The above-mentioned epoxy-phenol resin solution was used as thethermosetting paint. Nylon 12 having a softening point of 178° C. asdetermined according to the ring and ball method and a carbonyl groupconcentration of 508 meq per 100 g of the polymer was used as thethermoplastic resin. Pellets of the thermoplastic resin weremechanically pulverized to obtain a powder having an average particlesize of about 25μ. The powder was incorporated in the thermosettingresin solution so that the volume ratio as solids of both the resins wascontrolled as shown in Table 3. The mixture was stirred for 20 minutesby a high speed mixer to form a side seam-coating paint.

The contact angles of the so prepared paints were measured to obtain theresults shown in Table 3.

With respect to welded tinplate can bodies prepared according to theabove-mentioned method, the so prepared paint was spray-coated on theinner and outer surfaces of the seam portion of the can body (thicknessof the surface iron oxide layer=170-190 Å, thickness ratio of the alloylayer to the total plating thickness=55-70%) along a width of about 10mm by an airless spray gun while maintaining the paint temperature at40° to 70° C. during the spraying operation, so that the thickness ofthe coating after drying was 40 μm. Then, hot air was blown to thecoated can from below for about 10 seconds in the state where the sideseam on the inner face of the can was located above, whereby the solventwas substantially evaporated. The coated paint was then baked and curedin a gas oven maintained at 220° C. for about 60 seconds to obtain a canbody having a coated side seam.

The so obtained can body was tested with respect to items shown in Table3, and the structure of the coating layer of the welded can body wasobserved.

The so obtained can body having a coated side seam was subjected to beadprocessing, necked-in processing and flange processing, and a tinplatelid for a can having an inner diameter of 2 9/16 inches mm, which had anepoxy-phenol coating on each of the inner and outer surfaces, wasdouble-seamed to the processed can body. Consomme or tomato sauce waspacked in the obtained empty can, and the same tinplate lid as describedabove was double-seamed to the packed can by a vacuum seamer. Thesepacked cans were subjected to heat sterilization at 118° C. for 90minutes. Each packed can was stored at 37° C. for 1 year.

With respect to each sample, 100 cans were prepared and tested. Theamount of iron dissolved out and the amount of formed hydrogen weredetermined with optionally chosen 10 cans, and 50 cans optionally chosenfrom the remaining cans were opened and the corrosion state of the seamportion of the can body was examined.

The obtained results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________        Volume Ratio                                                                  of Thermo-             Side Seam-         State of                            plastic Resin                                                                        Contact         Covering                                                                              Processa-  Necked-In                       Sample                                                                            to Thermoset-                                                                        Angle                                                                              Dispersion Property                                                                              bility Adhe-                                                                             Processed                       No. ting Resins                                                                          (degrees)                                                                          State      (mA/300 mm.sup.2)                                                                     (mA/18 mm.sup.2)                                                                     sion                                                                              Portion                         __________________________________________________________________________    Samples of Present Invention                                                  12   7/93  23.6 nylon particles                                                                          0       0      O   no change                                       having particle                                                               size of about 22μ                                                          uniformly dis-                                                                persed in con-                                                                tinuous phase of                                                              thermosetting resin                                           13  20/80  29.0 nylon particles                                                                          0       0      O   no change                                       having particle                                                               size of about 22μ                                                          uniformly dis-                                                                persed in con-                                                                tinuous phase of                                                              thermosetting resin                                           14  40/60  36.2 nylon                                                                         particles having                                                                         0       0      O   no change                                       particle size of about                                                        23μ  dispersed in con-                                                     tinuous phase of ther-                                                        mosetting resin with                                                          particle blocking                                             15  85/15  30.1 thermosetting resin                                                                      0       0      O   no change                                       particles having parti-                                                       cle size of 5-20μ dis-                                                     persed in continuous                                                          phase of nylon                                                16  98/2   20.1 Thermosetting resin                                                                      0       0      O   no change                                       particles having                                                              particle size of 2-8μ                                                      dispersed in con-                                                             tinuous phase of nylon                                        __________________________________________________________________________                   Packed Can Test Results                                                       consomme           tomato sauce                                               amount (ppm)       amount                                                     of iron      number of                                                                           (ml/can)    number of                                  Sample                                                                            dissolved                                                                            state of                                                                            perforated                                                                          of formed                                                                           state of                                                                            perforated                                 No. out    side seam                                                                           cans  hydrogen                                                                            side seam                                                                           cans                            __________________________________________________________________________               Samples of Present Invention                                                  12  0.2    no change                                                                           0     0.15  no change                                                                           0                                          13  0.4    no change                                                                           0     0.14  no change                                                                           0                                          14  0.3    no change                                                                           0     0.17  no change                                                                           0                                          15  0.2    no change                                                                           0     0.16  no change                                                                           0                                          16  0.3    no change                                                                           0     0.21  no change                                                                           0                               __________________________________________________________________________        Volume Ratio                                                                  of Thermo-            Side Seam-          State of                            plastic Resin                                                                         Contact       Covering                                                                              Processa-   Necked-In                       Sample                                                                            to Thermoset-                                                                         Angle                                                                              Dispersion                                                                             Property                                                                              bility      Processed                       No. ting Resins                                                                           (degrees)                                                                          State    (mA/300 mm.sup.2)                                                                     (mA/18 mm.sup.2)                                                                     Adhesion                                                                           Portion                         __________________________________________________________________________    Comparative Samples                                                           17  0/100   18.1 --       2.5     8.4    O    peeled                          18  77/23   38.1 both nylon and                                                                         1.1     2.1    O    no change                                        thermosetting                                                                 resin formed                                                                  continuous                                                                    phases, uneven                                                                composition                                                  19  100/0   16.5 --       0       0      Δ                                                                            no change                       __________________________________________________________________________                    amount                                                                        (ppm) of           amount                                                     iron         number of                                                                           (ml/can)   number of                                   Sample                                                                            dissolved                                                                          state of                                                                              perforated                                                                          of formed                                                                           state of                                                                           perforated                                  No. out  side seam                                                                             cans  hydrogen                                                                            side seam                                                                          cans                            __________________________________________________________________________                Comparative                                                                   Samples                                                                       17  13.2 spot corrosion                                                                        7     some cans                                                                           spot 20                                                                 swollen                                                                             corrosion                                        18  6.2  spot corrosion                                                                        0     2.3   spot cor-                                                                     rosion in                                                                     some cans                                        19  5.1  blister 1     swollen                                                                             blister                                                                            5                                                                  cans                                       __________________________________________________________________________

EXAMPLE 4

In samples other than sample No. 24, the above-mentioned epoxy-urearesin solution (II) was used as the thermosetting paint, and theabove-mentioned epoxy urea resin solution (I) was used in sample No. 24.A polybutylene terephthalate (PBT)/polybutylene isophthalate (PBI)copolymer having a PBT content of 80 mol %, a carbonyl groupconcentration of 909 meq/100 g of the polymer and a softening point of205° C. as determined according to the ring and ball method was used asthe thermoplastic resin. In samples Nos. 22, 23 and 25, pellets of thethermoplastic resin were mechanically pulverized at normal temperaturesand then sieved. In samples Nos. 21 and 24, pellets of the thermoplasticresin were frozen with liquefied nitrogen and pulverized, and particleshaving an average particle size of about 15μ were used. In sample 20,pellets of the thermoplastic resin were dissolved in a solvent at a hightemperature and the temperature of the solution was gradually lowered toprecipitate particles. The resin powder was incorporated in the abovepaint so that the volume ratio as solids of the powder was 25%. Themixture was stirred by a high speed mixer for 20 minutes to form a sideseam coating paint.

The contact angles of the so formed paints were determined according tothe above-mentioned method to obtain the results shown in Table 4.

The above-mentioned paint was spray-coated on the inner and outersurfaces of the side seam portion of a TFS can body obtained accordingto the above-mentioned method (having a magnetite layer thickness of220-250 Å) along a width of about 10 mm by an airless spray gun whilemaintaining the paint temperature at 40° to 70° C. during the sprayingoperation, so that the thickness of the coating after drying was 40 to60 μm. Then, hot air was blown to the coated can from below for about 10seconds in the state where the side seam on the inner face of the canwas located above, whereby the solvent was substantially evaporated. Thecoated paint was then baked and cured in a gas oven maintained at 220°C. for about 30 seconds to obtain a can body having a coated side seam.

The so obtained can body was tested with respect to items shown in Table4, and the state of the thermoplastic resin powder dispersed in thecured coating was observed.

The so obtained can body having a coated side seam was subjected toflange processing, and a TFS lid for a can having an inner diameter of 211/16 inches, which had an epoxy-phenol coating on each of the inner andouter surfaces, was double-seamed to the processed can body. Consomme ortomato sauce was packed in the obtained empty can, and the same TFS lidas described above was double-seamed to the packed can by a vacuumseamer. The packed can was subjected to the heating sterilization at118° C. for 90 minutes. Each packed can was stored at 37° C. for 1 year.

With respect to each sample, 100 cans were prepared and tested. Theamount of iron dissolved out and the amount of formed hydrogen weredetermined with optionally chosen 10 cans, and 50 cans optionally chosenfrom the remaining cans were opened and the corrosion state of the seamportion of the can body was examined.

The obtained results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________        Average                                                                       Particle                                                                      Size (mμ)                                                                  of Thermo-                                                                          Contact                Side Seam-                                                                            Processa-                            Sample                                                                            plastic                                                                             Angle                  Covering                                                                              bility Adhe-                         No. Resin (degrees)                                                                          Dispersion State  (mA/300mm.sup.2)                                                                      (mA/18mm.sup.2)                                                                      sion                          __________________________________________________________________________    Samples of Present Invention                                                  20  1.1   35.1 thermoplastic resin particles having                                                            0       0      O                                            particle size of about 1μ dispersed                                        in continuous phase of thermosetting                                          resin substantially uniformly                                  21  15    30.3 thermoplastic resin particles having                                                            0       0      O                                            particle size of about 13μ dispersed                                       in continuous phase of thermosetting                                          resin substantially uniformly                                  22  50    27.2 thermoplastic resin particles having                                                            0       0      O                                            particle size of about 40μ dispersed                                       in continuous phase of thermosetting                                          resin                                                          23  70    24.1 thermoplastic resin particles having                                                            0       0      O                                            particle size of about 50μ dispersed                                       in continuous phase of thermosetting                                          resin                                                          Comparative Samples                                                           24  15    18.6 substantially homogeneous phase, pre-                                                           0.3     0.3    O                                            sence of thermoplastic resin particles                                        not confirmed                                                  25  100   18.6 continuous phase of thermosetting                                                               0.5     5.1    O                                            resin formed but dispersion state                                             of thermoplastic resin particles                                              not good                                                       __________________________________________________________________________                   Packed Can Test Results                                                       Consomme          tomato sauce                                                amount (ppm)      amount                                                      of iron     number of                                                                           (ml/can)     number of                                  Sample                                                                            dissolved                                                                            state of                                                                           perforated                                                                          of format                                                                          state of                                                                              perforated                                 No. out    side seam                                                                          cans  hydrogen                                                                           side seam                                                                             cans                            __________________________________________________________________________               Samples of Present Invention                                                  20  0.02   no change                                                                          0     0.04 no change                                                                             0                                          21  0.09   no change                                                                          0     0.05 no change                                                                             0                                          22  0.06   no change                                                                          0     0.08 no change                                                                             0                                          23  0.13   no change                                                                          0     0.10 no change                                                                             0                                          Comparative Samples                                                           24  1.24   spot 0     swollen                                                                            spot    14                                                    corrosion  cans corrosion                                          25  2.7    blister                                                                            0     swollen                                                                            spot corrosion                                                                        12                                                               cans in some cans                            __________________________________________________________________________

EXAMPLE 5

A powder of a thermoplastic resin shown in Table 5 was used, and theabove-mentioned epoxy-phenol type resin solution was used as thethermosetting resin paint. The powder was formed mainly by mechanicalpulverization to adjust the average particle size to a value shown inTable 5. The powder was incorporated into the thermosetting resinsolution so that the volume ratio as solids of the thermoplastic resinto the thermosetting resin was 85/15. At this step, the same solvent asused for the thermosetting resin solution was used as the diluent. Themixture was stirred by a high speed mixer to form a paint solution.

The contact angles on glass plate of these paints were not remarkablydependent on the kind of thermoplastic resins for sample Nos. 26-32within 26-33 degrees and 22 degrees for sample No. 33.

The above-mentioned paint was spray-coated on the inner and outersurfaces of the seam portion of the tinplate can body prepared accordingto the above-mentioned method along a width of about 10 mm by an airlessspray gun while maintaining the paint temperature at 40° to 70° C.during the spraying operation, so that the thickness of the coatingafter drying was 50 μm. Then, hot air was blown to the coated can frombelow for about 10 seconds in the state where the seam on the inner faceof the can was located above, whereby the solvent was substantiallyevaporated. The coated paint was then baked and cured in a gas ovenmaintained at 220° C. for about 60 seconds to obtain a can body having acoated side seam.

The so obtained can body was tested with respect to items shown in Table5, and the dispersion state of the thermoplastic resin powder in thecured coating was observed.

The so obtained can body having a coated side seam was subjected to beadprocessing, necked-in processing and flange processing, and a tinplatelid for a can having an inner diameter of 65.3 mm, which had anepoxy-phenol coating on each of the inner and outer surfaces, wasdouble-seamed to the processed can body. An apple drink heated at 90° C.or tomato sauce was packed in the obtained empty can, and the sametinplate lid as described above was double-seamed to the packed can by avacuum seamer. The can packed with the tomato source was subjected tothe heating sterilization at 118° C. for 90 minutes. Each packed can wasstored at 37° C. for 1 year.

With respect to each sample, 100 cans were prepared and tested. Theamount of iron dissolved out and the amount of formed hydrogen weredetermined with optionally chosen 10 cans, and 50 cans optionally chosenfrom the remaining cans were opened and the corrosion state of the sideseam portion of the can body was examined.

                                      TABLE 5                                     __________________________________________________________________________        Thermoplastic Resin                                                           Powder [softening                                                             point (° C.), carbonyl                                                 group concentration      Side Seam-          State of                         (meq/100 g of poly-      Covering                                                                              Processa-   Necked-in                    Sample                                                                            mer) average particle    Property                                                                              bility Adhe-                                                                              Processed                    No. size (μm)]                                                                            Dispersion State                                                                            (mA/300mm.sup.2)                                                                      (mA/18mm.sup.2)                                                                      sion Portion                      __________________________________________________________________________    Samples of Present Invention                                                  26  polybutylene tere-                                                                       thermoplastic resin parti-                                                                  0       0      O    no change                        phthalate (222,                                                                          cles having particle size of                                       909, 20)   about 18μ dispersed in con-                                                tinuous phase of thermoset-                                                   ting resin substantially                                                      uniformly                                                      27  polyethylene tere-                                                                       thermoplastic resin particles                                                               0       0      O    no change                        phthalate/polypro-                                                                       having particle size of about                                      pylene terephthalate                                                                     14μ dispersed in continuous                                     copolymer (205, 1028,                                                                    phase of thermosetting resin                                       15)        substantially uniformly                                        28  maleic acid-                                                                             thermoplastic resin particles                                                               0       0      O    no change                        modified poly-                                                                           having particle size of about                                      propylene  10μ dispersed in continuous                                     (165, 80, 10)                                                                            phase of thermosetting resin                                                  with partial blocking                                          29  polycarbonate                                                                            thermoplastic resin particles                                                               0       0      O    no change                        (150, 394, 20)                                                                           having particle size of about                                                 μ dispersed in continuous                                                  phase of thermosetting resin                                                  substantially uniformly                                        __________________________________________________________________________                      Packed Can Test Results                                                       apple drink        tomato sauce                                               amount (ppm)       amount                                                     of iron      number of                                                                           (ml/can)    number of                                  Sample                                                                            dissolved                                                                            state of                                                                            perforated                                                                          of formed                                                                           state of                                                                            perforated                                 No. out    side seam                                                                           cans  hydrogen                                                                            side seam                                                                           cans                         __________________________________________________________________________                  Samples of Present Invention                                                  26  5.2    no change                                                                           0     0.19  no change                                                                           0                                          27  4.7    no change                                                                           0     0.20  no change                                                                           0                                          28  6.3    no change                                                                           0     0.38  no change                                                                           0                                          29  6.2    no change                                                                           0     0.34  no change                                                                           0                            __________________________________________________________________________        Thermoplastic Resin                                                           Powder [softening                                                             point (°C.), carbonyl                                                  group concentration      Side Seam-          State of                         (meq/100 g of poly-      Covering                                                                              Processa-   Necked-in                    Sample                                                                            mer) average particle    Property                                                                              bility      Processed                    No. size (μm)]                                                                            Dispersion State                                                                            (mA/300 mm.sup.2)                                                                     (mA/13mm.sup.2)                                                                      Adhesion                                                                           Portion                      __________________________________________________________________________    Samples of Present Invention                                                  30  nylon 6/nylon 12                                                                         thermoplastic resin particles                                                               0       0      O    no change                        copolymer  having particle size of about                                      (140, 644, 30)                                                                           25μ dispersed in continuous -                                                                    phase of thermosetting resin                            substantially uniformly                                        Comparative Samples                                                           31  ethylene-propylene                                                                       prominent blocking of                                                                       0.1     0.3    Δ                                                                            no change                        copolymer  thermoplastic resin                                                (-30, 0, 50)                                                                             particles                                                      32  polyethylene                                                                             localized distribution of                                                                   0       0.2    Δ                                                                            partially                        (105, 0, 50)                                                                             thermoplastic resin particles     peeled                                      on surface of coating,                                                        prominent blocking                                             33  polyvinyl  substantially homogeneous                                                                   2.1     0.5    Δ                                                                            no change                        chloride   phase, presence of thermo-                                         (90, 0, 5) plastic resin particles                                                       not confirmed                                                  __________________________________________________________________________                  Packed Can Test Results                                                       apple drink          tomato sauce                                             amount (ppm)         amount                                                   of iron        number of                                                                           (ml/can)      number of                              Sample                                                                            dissolved                                                                            state of                                                                              perforated                                                                          of formed                                                                           state of                                                                              perforated                             No. out    side seam                                                                             cans  hydrogen                                                                            side seam                                                                             cans                         __________________________________________________________________________              Samples of Present Invention                                                  30  4.2    no change                                                                             0     0.33  no change                                                                             0                                      Comparative Samples                                                           31  13.2   spot corrosion                                                                        0     5.2   spot corrosion                                                                        3                                                 in some cans        substantiallly                                                                on entire                                                                     surface                                        32  8.8    spot corrosion                                                                        0     6.2   spot corrosion                                                                        2                                                 in some cans        substantially                                                                 on entire                                                                     surface                                        33  19.3   spot corrosion                                                                        0     3.3   spot corrosion                                                                        6                                                 in some cans        in some cans                         __________________________________________________________________________

EXAMPLE 6

A powder having an average particle size of about 20 μm, which wasobtained by mechanically pulverizing pellets of nylon 11 having asoftening point of 185° C. and a carbonyl group concentration of 546 meqper 100 g of the polymer, was used as the thermoplastic resin powder,and the above-mentioned epoxy-urea type resin solution (II) was used asthe thermosetting resin solution. By using the same diluent solvent asused for the thermosetting resin solution, both the resins were mixed bya high speed mixer to obtain a paint having a solid content of about25%, in which the ratio of the thermoplastic resin to the thermosettingresin in the solids was 20/80.

An epoxy-urea type paint (a 3/1 weight ratio mixture of an epoxy resinand a butylether-urea/formaldehyde resin) was coated, baked and cured ona tinplate having a thickness of 0.24 mm according to theabove-mentioned method, and the coated tinplate was cut into a bodyblank having a length of 200.40 mm and a height of 125.40 mm, and awelded tinplate can body was prepared in a nitrogen current supplied ata flow rate of 5 l/min. When the surface metal layer of the weldedportion was analyzed according the above-mentioned method, it was foundthat the thickness of the iron oxide layer was 180 to 200 Å and theratio of the thickness of the alloy layer to the total plating thicknesswas 40 to 60%.

The above-mentioned paint was spray-coated on the inner and outersurfaces of the side seam portion of the can body along a width of about10 mm by an airless spray gun while maintaining the paint temperature at40° to 70° C. during the spraying operation, so that the thickness ofthe coating after drying was 50 μm. Then, hot air was blown to thecoated can from below for about 10 seconds in the state where the seamon the inner face of the can was located above, whereby the solvent wassubstantially evaporated. The coated paint was then baked and cured in agas oven maintained at 220° C. for about 30 seconds to obtain a can bodyhaving a coated side seam.

The structure of the coating layer in the side seam portion was observedaccording to the above-mentioned method. It was found that thermoplasticresin particles having a particle size of about 17 μm were dispersed inthe continuous phase of the thermosetting resin. The covering propertyand processability were examined according to the above-mentionedmethods. It was found that both the non-processed portion and theportion subjected to the bending processing were completely covered andbreaks or cracks were not formed by the processing.

The so obtained can body was subjected to necked-in processing andflange processing, and a tinplate lid for a can having an inner diameterof 2 9/16 inches and a domed top having an epoxy-phenol type coating oneach of the inner and outer surfaces were double-seamed to the can bodyfor aerosol. A glass cleaner or washing starch was packed in this emptycan according to customary procedures. A mounting cap was attached andthe packed can was stored at 45° C. for 3 or 6 months, and the change ofthe weight of the content was checked. With respect to each sample, 50cans were opened and the absence or presence of corrosion in the sideseam portion of the can body was examined. In each case, leakage orcorrosion of the side seam portion was not observed at all.

What we claim is:
 1. A process for the preparation of side seam-coatedwelded cans, which comprises applying a resin paint to a welded can bodyhaving a seam formed on the side face thereof at least on one surface ofsaid seam and baking the coated paint to form a coating covering saidside seam, wherein said resin paint comprises a dispersion mediumcomposed of a solution of a thermosetting resin and a dispersed phase ofresin particles having a number average particle size of 0.1 to 80 μmand being composed of a thermoplastic resin having a softening point of50° to 300° C. as determined according to the ring and ball method, thevolume ratio of the thermosetting resin to the thermoplastic resin inthe coating ranges (A) from 95/5 to 25/75 wherein the thermosettingresin forms the continuous phase and the thermoplastic resin forms thedispersed phase or (B) from 20/80 to 1/99 wherein the thermoplasticresin forms the continuous phase and the thermosetting resin forms thedispersed phase, and the paint coated on the side seam is baked undersuch conditions that evaporation of a solvent in the solution of thedispersion medium is first caused and softening of the thermoplasticresin is then caused, the thermoplastic polymer containing in the mainchain or side chain a carboxyl group derived from a carboxylic acid, acarboxylic acid salt, a carboxylic acid anhydride, a carboxylic acidester, a carboxylic acid amide, a carbonic acid ester, urea, orurethane, the carboxyl groups having a concentration of 12 to 1400 megper 100 g of the polymer.
 2. A process according to claim 1 wherein saidresin paint has a contact angle of 18.5° to 45° when said paint isdropped on a glass sheet at 25° C. and allowed to stand 1 minute.
 3. Aprocess accoring to claim 1 wherein said resin paint has a solidconcentration of from 5 to 50%.
 4. A process according to claim 1wherein the paint coated on the side seam is baked at a temperature of150° to 400° C. for 1 second to 20 minutes.
 5. A process according toclaim 1 wherein the thermoplastic resin is a polyester, polycarbonate,polyamide, ionomer or acid-modified polyolefin.
 6. A process accordingto claim 1 wherein the thermosetting resin is a combination of an epoxyresin with at least one member selected from the group consisting ofphenolic resins, urea resins, melamine resins and thermosetting acrylicresins.